15" Fossil Mosasaur Skull Section - Goulmima, Morocco

About Mosasaurs

Mosasaurs were large, carnivorous marine reptiles that lived during the Late Cretaceous Period, approximately 90 to 66 million years ago. Though often mistaken for dinosaurs, they were actually more closely related to modern monitor lizards, including the Komodo dragon. Over time, their ancestors transitioned from land to sea, evolving paddle-like flippers, streamlined bodies, and powerful tails that made them highly efficient swimmers. Some species developed crescent-shaped, shark-like tail fins that allowed for strong, sustained propulsion through open water. Sizes varied widely, with many species measuring 20 to 30 feet long, while the largest—such as Mosasaurus hoffmannii—may have exceeded 50 feet in length.

As apex predators, mosasaurs occupied the top of the marine food chain. Their long jaws were lined with sharp, conical teeth ideal for gripping slippery prey, and fossil evidence shows they fed on fish, sharks, ammonites, seabirds, and even other mosasaurs. Despite their fully aquatic lifestyle, mosasaurs were air-breathing reptiles and had to surface regularly, much like modern whales. They thrived in oceans around the world until the Cretaceous–Paleogene extinction event 66 million years ago, which wiped out the dinosaurs and many marine reptiles, bringing the age of mosasaurs to an end.

Tethysaurus was a small-to-medium-sized mosasaur that lived during the Early Turonian stage of the Late Cretaceous period, approximately 93 million years ago. It grew to around 10 feet (3 meters) in length and seemed to be a transitional species between some of the more primitive mosasaurs and later mosasaur groups. Its fossils have been found near the villages of Asfla and Tadirhourst in east-central Morocco.

Tethysaurus was named after Tethys,‭ ‬the ancient Greek goddess of the sea. The ancient Tethys Ocean, which once separated Laurasia‭ (‬the northern continents‭) ‬from Gondwana‭ (‬the southern continents‭), is also named for this Greek goddess.